#include <iostream>
#include <string>
using namespace std;

bool isPrime(int n) {
    if (n <= 1) {
        return false;
    }
    for (int i = 2; i * i <= n; ++i) {
        if (n % i == 0) {
            return false;
        }
    }
    return true;
}
int main() {
        int number;
        cin >> number;
        
        if (number < 1000 || number > 9999) {
            cout << "0";
            return 1;
        }
        int max = 0;
       
        string n = to_string(number);
        int x = n.length(); 

       
        for (int i = 0; i < x; ++i) {
            
            for (int k = x; k > i; --k) {

                
                int num = stoi(n.substr(i, k - i));
               
                if (isPrime(num) && num > max) {
                    max = num;
                }
            }
        }
        cout << max;

        return 0;
    }
    